Agriculturally active materials, such as pesticides, herbicides and the like, are widely used throughout the agricultural industry. For convenience in packaging and handling, such agriculturally active materials are typically produced in the form of a dry solid, such as a powder, which can be readily mixed with water. The aqueous solution or dispersion of the active material is then typically applied to an area to be treated by spraying.
In order to produce aqueous solutions or dispersions suitable for application by spraying, the agriculturally active material must be in a form which can be readily incorporated with water. The agriculturally active material by itself, however, is usually insoluble in water or has an unacceptably low water solubility. Therefore, it is usually necessary to treat the agriculturally active material in some manner to enhance its combinability with water.
One such method of treating agriculturally active materials is microencapsulation. For example, U.S. Pat. No. 4,280,833 discloses a process for microencapsulating water-immiscible materials, such as herbicides, and, specifically, trifluralin. The microencapsulation process involves an aqueous phase containing an emulsifier and an organic phase. The organic phase consists of a water-immiscible material, such as trifluralin, and polymethylene polyphenylisocyanate. The organic phase is added to the aqueous phase with agitation to form a dispersion of small droplets of the organic phase within the aqueous phase. Thereafter, a polyfunctional amine is added to the dispersion. The polyfunctional amine reacts with the isocyanate to form a capsular polyurea shell about the herbicide droplet. This type of microencapsulation process is termed interfacial polycondensation. Other patents which involve microencapsulation by polycondensation processes include U.S. Pat. Nos. 4,360,376; 4,417,916; 4,563,212; 3,429,827; 3,577,515; 3,959,464 and 4,640,709.
Another process used to microencapsulate active materials is spray drying. U.S. Pat. No. 4,244,836 relates to a microencapsulation process using spray drying. In that process, a liquid, water-insoluble phase is dispersed in an aqueous phase. The liquid, water-insoluble phase can be materials, such as plant protecting agents. The aqueous phase is a water-soluble polyvinyl alcohol solution. The water-insoluble phase is dispersed in the aqueous phase using a stirrer or a homogenization device so as to produce droplets of the water-insoluble phase of from 1 to 50 microns in diameter within the aqueous phase. The dispersion in then atomized into a stream of heated air (spray dried). The spray drying dehydrates the aqueous dispersion and produces a dry powdery microcapsular product. Other patents which utilize a spray drying technique for microencapsulation include U.S. Pat. Nos. 4,286,020; 4,353,962 and 4,690,786.
Active materials which can be microencapsulated include plant protecting agents, such as herbicides. Many conventional herbicides, particularly dinitroaniline-based herbicides, contain nitrosamine impurities. Since nitrosamines are recognized carcinogens and since current governmental regulations limit the amount of nitrosamines which herbicides can contain, it is desirable to reduce the level of nitrosamines present in herbicides, particularly trifluralin. Various methods are known for reducing the level of nitrosamine impurities in herbicides as shown in U.S. Pat. Nos. 4,335,260; 4,338,473; 4,440,962; 4,501,608; and 4,537,992. These known methods of reducing nitrosamine contamination, however, typically involve relatively costly and complicated chemical processes. Accordingly, a relatively simple, effective and inexpensive method for reducing nitrosamine impurities in herbicides has long been sought.
Some herbicides are known to exist in two or more polymorphic forms. For example, U.S. Pat. No. 4,082,537 discloses that N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine [also known as N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] exists in two distinct polymorphic forms: a yellow microcrystalline form and an orange macrocrystalline form. The yellow polymorph, although considered less stable than the orange polymorph, has the advantage of being slower to settle out of aqueous dispersions, such as those involved with spraying equipment. The yellow polymorph is found in freshly prepared N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine which slowly converts to the orange polymorph with age. However, when sodium dioctyl sulfosuccinate is added to molten N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine followed by cooling, solidification and conversion to a wetable powder, the compound produces stable aqueous dispersions of the yellow polymorph. The production of herbicidal dispersions which are more stable than the known herbicidal dispersions is therefore desirable.